A thermopile is an infrared radiation (IR) detector commonly used for making non-contact temperature measurements. For example, thermopiles are used in ear thermometers, proximity sensors, heat flux sensors, and the like. Thermopiles are made up of a series of electrically connected thermocouple pairs, each pair made up of dissimilar conducting or semi-conducting materials with different Seebeck coefficients. For example, N-type poly-silicon and P-type poly-silicon are often used in conventional thermopiles.
Generally, one end of each thermocouple is placed in contact with a membrane operable to collect IR energy while the other end is placed on a supporting substrate. The collected IR energy creates a temperature gradient across the thermocouple, causing the thermocouple to generate an output voltage via the Seebeck effect. For a thermocouple having known characteristics, the output voltage may be converted into a temperature value.
However, since the output voltage of a thermocouple is relatively small, efforts have been made to provide designs and methods that are capable of maximizing the heat trapped within the membrane and therefore enhance the signal. Such efforts have included the use of vacuum packaging, significantly increasing the membrane area, as well as providing a suspended (“released”) membrane to provide for thermal isolation. For example, one typical process involves using low-pressure chemical-vapor deposition (LPCVD) methods to deposit a membrane material (e.g., silicon nitride, polycrystalline silicon) onto a substrate (e.g., silicon) after depositing a sacrificial layer (e.g., an LPCVD or thermally grown silicon oxide layer). The sacrificial layer is later removed by wet etching via etch holes to thereby create a cavity over which the sensor is suspended and supported by the membrane. This suspension process can also be referred to as “releasing” the membrane. In addition, because the heat from the membrane can dissipate through the air surrounding the exposed membrane, a vacuum may be created within the cavity to further reduce heat loss via conduction and convection.
While such sensor designs provide the desired enhanced signals, very extensive process steps are necessary in order to form the suspended membrane, which generally requires eight or more mask layers within a CMOS process. This comes along with a significant decrease of yield and an increase of manufacturing time and costs. In addition, the suspended membrane is fragile and may be prone to tearing and damage due to handling, thus resulting in further decreased yield and increased manufacturing time and costs.
Non-suspended (“unreleased”) thermopile IR sensors have been developed, but they have suffered from a low Figure of Merit (FOM) due to the low temperature budget of the utilized thermally isolating materials which can be used for IR sensing. In addition, proposed materials that could potentially offer enhanced sensitivity are generally not CMOS compatible due to their instability at the heightened temperatures at which CMOS is carried out. As such, the suspended sensor and membrane structure has generally been the standard in achieving heightened sensitivity.
It would be desirable to overcome the complexity of the manufacturing process for thermopile IR sensors, thereby reducing manufacturing time and expense and increasing yield. It would further be desirable to provide fabrication methods and sensor designs which allow for more structurally stable configurations that are less prone to damage, while still achieving the necessary enhanced signals.